Drink dispensers, in particular automatic coffee machines, include tubes for transporting hot or cold liquid foods. It is known to use, for these tubes, silicones or polyorganosiloxanes, a material which is distinguished by its longevity, its flexibility, its relatively limited cost, its high chemical inertia and its good resistance to high and low temperatures.
However, polyorganosiloxanes exhibit the disadvantage of being hydrophobic, which presents a problem when it is desired to use them for tubes intended to transport liquids which include proteins, such as milk.
When proteins, in suspension or in solution in an aqueous liquid, come into contact with a hydrophobic surface, they are adsorbed, often irreversibly, and are difficult, indeed even impossible, to remove by simple rinsing. The fouling of the tubes by protein films presents problems of a detrimental change in the taste and of bacterial proliferation.
It is known to reduce the hydrophobic nature of liquid silicones (LSRs) by incorporation of polydimethylsiloxane-poly(ethylene glycol) (PDMS-PEG or PDMS-PEO) copolymers, in particular with the aim of limiting or preventing the adsorption of proteins and the fouling which ensues.
U.S. Pat. No. 8,053,548, for example, describes the preparation of hydrophilic silicones by mixing a PDMS base, a crosslinking agent and amphiphilic PDMS-PEO block copolymers. Figure 3 shows the decrease in the contact angle as a function of the amount of amphiphilic copolymer added.
The paper entitled “Hydrophilic PEO-PDMS for microfluidic applications” by Yao et al. also describes the preparation of PDMS sheets, the surface of which has been rendered hydrophilic by incorporation of an amphiphilic PDMS-PEO block copolymer.
In both cases, the silicone base used is of liquid (LSR) type. Specifically, silicones which cure at high temperature (HTV, High Temperature Vulcanizing) are conventionally divided into two categories:
LSRs (Liquid Silicone Rubbers) are more or less fluid elastomer bases which flow freely or at least can be pumped. They are sold in the form of two-component kits including an elastomer base formed by monomers or prepolymers of low molecular weight and of a reinforcing filler (component A) and a curing catalyst, generally platinum-based (component B), which catalyzes a hydrosilylation reaction between a vinyl functional group and a hydrosilyl functional group. LSRs are conventionally used for the injection moulding of complex pieces.
HCRs (High Consistency Rubbers), on the other hand, are elastomer bases of very high viscosity, generally of greater than 106 mPa·s. They are generally sold in the form of single-component compositions including a polyorganosiloxane of high weight including vinyl groups, a reinforcing filler and a curing agent (peroxide). The heating or the irradiation of the composition triggers the decomposition of the curing agent (radical initiator or photoinitiator) and the polymerization of the vinyl groups.
However, there also exist two-component HCRs which cure by platinum-catalyzed hydrosilylation.
In the two documents of the state of the art mentioned above, the elastomer bases are of two-component LSR type. The weakly to moderately viscous nature of these resins allows the amphiphilic copolymer to migrate, before crosslinking, towards the surface of the piece where the molecules become oriented so that their hydrophilic part is turned towards the surface, while the PDMS part becomes anchored in the body of the PDMS base. A period of venting of several tens of minutes, indeed even of several hours, is thus provided in the two abovementioned documents in order to allow the hydrophilizing surfactant to accumulate at the surface of the composition.
Unfortunately, such liquid bases are not appropriate for the extrusion of silicone tubes or profiled elements. This is because the low viscosity of the LSRs makes difficult the shaping by extrusion of tubes or profiled elements which are sufficiently rigid to retain their shape up to the crosslinking stage.
Furthermore, in the two abovementioned documents of the state of the art, the LSR mixture, once vented, is cured at a temperature of the order of 60° C. to 80° C. for more than one hour. Such a long heating at a relatively low temperature is not appropriate either for an extrusion process.
To the knowledge of the Applicant Company, the incorporation of amphiphilic copolymers of PDMS-PEO type in HCRs, for the purpose of an extrusion process, has until now never been described.
This is because a person skilled in the art could assume that the virtually solid consistency of such elastomer bases would prevent the migration of the copolymers towards the surface of the extruded tubes and that the amounts of amphiphilic copolymer necessary in order to obtain a significant hydrophilizing effect would for this reason be very high.
In point of fact, the incorporation of high amounts, for example of more than 5% by weight, of amphiphilic copolymer is not desirable for the following reasons:
it might result in an increase in the production cost of the pieces produced;
a deterioration in the mechanical properties of the finished pieces might result therefrom, for example a decrease in the Young's modulus and in the elongation at break;
the duration of the crosslinking stage would be extended, which, at high temperatures (>200° C.) often necessary in the context of an extrusion process, might present problems of chemical decomposition of the hydrophilic part of the amphiphilic copolymer and might greatly reduce the overall speed of the production line.